Multifilar helix antennas

ABSTRACT

A multifilar helix antenna comprises a plurality of helical antenna filaments each having a square waveform pattern along its length, giving reduced axial length. Also described is a multifilar helix antenna comprising a plurality of helical antenna filaments each incorporating a microstrip spur-line band stop filter for enabling multi-band operation.

The invention relates to multifilar helix antennas, particularly, thoughnot exclusively, quadrifilar helix antennas.

The quadrifilar helix antenna (QHA) has been widely advocated for use,inter alia, in mobile satellite communications systems. Compared withcrossed dipole and patch antennas, the QHA offers the advantages that ithas a small, compact structure, is relatively insensitive to the effectsof handling and of the ground and has a radiation pattern and a widecircularly polarised beam that can be readily shaped. The so-calledprinted QHA (PQHA) is particularly advantageous because of its lightweight, low cost, high dimensional stability and ease of fabrication.

Although existing PQHA structures are already quite small, further sizereduction is still required to satisfy the space limitations in handheldmobile communications terminals.

Various approaches have been adopted with a view to reducing thephysical size of a QHA. One approach involves loading the QHA with adielectric material such as Zirconium Tintinate ceramic. Although thisgives significant size reduction the operating bandwidth of the antennais very small, typically about 30 MHz which is unsatisfactory for manymobile communications applications.

A coupled-segment QHA has also been proposed. In this case the helicalantenna filaments are separated into upper and lower segments which areinterleaved in overlapping fashion. This approach only provides a smallpercentage of size reduction.

According to one aspect of the invention there is provided a multifilarhelix antenna comprising a plurality of helical antenna filaments spacedapart from each other at regular intervals about a longitudinal axis ofthe antenna, each said helical antenna filament having a meander alongits length.

Preferably, the meander is periodic and may have a rectangular waveformshape.

In a preferred embodiment the multifilar helix antenna is a printedmultifilar helix antenna. Said periodic meander preferably has a squarewaveform pattern.

As mobile communications systems evolve there is now an urgent need formobile communications antennas capable of operating over multiplerelatively wide frequency bands and yet are compact and light weight.

One known dual band QHA comprises two tuned helix antennas, one insideanother or a monopole antenna (which may be wound) placed inside a helixantenna and tuned to a higher frequency, and yet another known dual bandhelix antenna comprises a helix antenna and a separate parasiticelement. Another known dual band QHA comprises a single helix antennahaving an increasing or a decreasing pitch angle, and in yet anotherarrangement PIN diodes are provided to short circuit segments of thehelical antenna filaments creating an antenna having two differentresonant frequencies. These known antennas have complex structures andare difficult and expensive to manufacture in practice.

According to another aspect of the invention there is provided amultifilar helix antenna comprising a plurality of helical antennafilaments spaced apart from each other at regular intervals about alongitudinal axis of the antenna and wherein each said helical antennafilament incorporates a band stop filter for enabling multi-bandoperation.

The band stop filter is preferably a microstrip spur-line band stopfilter.

Said one and another aspects of the invention may be implemented incombination.

Embodiments of the invention are now described, by way of example only,with reference to the accompanying drawings in which:

FIG. 1(a) shows a planar representation of the helical filaments of aMPQHA according to one aspect of the invention,

FIG. 1(b) shows a planar representation of a conventional PQHA,

FIG. 2(a) shows a plot of antenna return loss as a function of frequencyobtained for a reference PQHA,

FIG. 2(b) shows the radiation pattern obtained for the reference PQHA atfour different frequencies,

FIG. 3 shows plots of frequency F as a function of ΔA obtained fordifferent implementations of MPQHA,

FIG. 4(a) shows a plot of antenna return loss as a function of frequencyobtained for an optimised MPQHA,

FIG. 4(b) shows the radiation pattern obtained for the optimised MPQHAat four different frequencies,

FIG. 4(c) shows radiation patterns obtained for the optimised MPQHA andfor the reference PQHA at 2 GHz,

FIG. 5(a) shows plots of antenna return loss as a function of frequencyobtained using optimised values of ΔL,

FIG. 5(b) shows radiation patterns obtained using the optimised valuesof ΔL and for the reference PQHA, at 2 GHz,

FIG. 6 shows a planar representation of the helical filaments of a MPQHAaccording to a further aspect of the invention,

FIG. 7 is a schematic view of a section of filament track containing aband stop filter,

FIG. 8(a) is a plot of antenna gain as a function of frequency obtainedfor a MPQHA with and without a microstrip spur-line band stop filter,

FIG. 8(b) shows radiation patterns for a DB MPQHA according to saidfurther aspect of the invention,

FIG. 9 shows the respective positions of the DSC 1800 and UMTS frequencyranges in the plot of FIG. 8(a),

FIG. 10 shows radiation patterns obtained for the DB MPQHA at fourdifferent frequencies,

FIG. 11 shows plots of antenna return loss as a function of frequencyfor a PQHA according to the further aspect of the invention.

The inventors have discovered that the axial length of a multifilarhelix antenna can be significantly reduced, without substantial loss ofperformance, if each filament of the antenna is provided with a periodicmeander along its length.

Preferably, the meander has a rectangular waveform pattern, andpreferred embodiments of the invention will now be described, by way ofexample, with reference to a meander printed quadrifilar helix antennain which each filament has a square waveform pattern; that is, arectangular waveform pattern having a mark-to-space ratio of unity.These embodiments will be referred to hereinafter as MPQHA.

FIG. 1(a) of the drawings shows a planar representation of the MPQHA andFIG. 1(b) shows a planar representation of a corresponding conventionalprinted quadrifilar helix antenna (PHQA).

In practice, each filament of the MPQHA consists of a track formed byprinting on an outer surface of a cylindrical substrate. The tracksfollow helical paths, and are spaced apart from each other at regularintervals about the longitudinal axis of the substrate.

Each periodic element 10 of the meandered filaments has a length 2ΔL anda width W=ΔA+w, where ΔA is the height of the square waveform patternand w is the width of the track, and so the total length of eachfilament is 2n(ΔL+ΔA), where n is the number of elements in thefilament.

As described in “Antenna Design for the ICO Handheld Terminal” by AgiusA. A. et al, 10th International Conference on Antennas and Propagation,14-17 April 1997, Conference Publication, No. 436, IEE 1997, the totallength L_(fil) of each filament of a conventional PQHA can be related tothe axial length Laxial by the expression:$L_{fil} = {N\sqrt{\left( \frac{Laxial}{N} \right)^{2} + \left( {2\pi\quad r} \right)^{2}}}$where r is the radius of the PQHA.

In analgous fashion, it can be shown that the axial length Laxial(MPQHA). of the MPQHA can be expressed as:${{Laxial}({MPQHA})} = {{N\sqrt{\left( \frac{2n\quad\Delta\quad L}{N} \right)^{2} - \left( {2\pi\quad r} \right)^{2}}\quad{for}\quad\left( \frac{2n\quad\Delta\quad L}{N} \right)^{2}} > \left( {2\pi\quad r} \right)^{2}}$

As shown in FIGS. 1(a) and 1(b), for comparable filaments, having thesame total length, the axial length of the MPQHA is significantly lessthan the axial length of the conventional PQHA, and the size reductionfactor α can be defined as:$\alpha = \frac{{{Laxial}({PQHA})} - {{{Laxial}({MPQHA})} \times 100\%}}{{Laxial}({PQHA})}$

The values selected for ΔL and ΔA will affect both the physical size andthe frequency response characteristic of the antenna. However, thegeometry of a quadrifilar helix antenna does impose certain constraintson the range of values that can be used in practice. In particular,neighbouring filments must not touch or overlap each other, and thisimposes an upper limit on the value of ΔA. This upper limit, ΔA_(max)can be expressed as:${\Delta\quad A_{\max}} = {\left( {\frac{2\pi\quad r}{4}\sin\quad\phi} \right) - w}$where φ is the pitch angle of the MPQHA.

Also, the value of ΔL has a lower limit ΔL_(min) given by:ΔL _(min) =w+1where ΔL_(min) and w are both expresed in millimetres.

In order to assess the physical and operational characteristics of theMPQHA, the axial length and resonant frequency of each of a wide rangeof different implementations of the MPQHA was compared with the axiallength and resonant frequency of a reference PQHA. The PQHA chosen forthis purpose had the following geometric parameters: Axial length,Laxial (PQHA)    83 mm Total Filament Length, L_(fil) 89.315 mm Antennaradius, r    7 mm Track width w    2 mm Number of Turns, N 0.75 ResonantFrequency, F    2 GHz

Each implementation of the MPQHA used in the comparison had the samevalues of L_(fil) (89.315 mm), r (7 mm), w (2 mm) and N (0.75).

Table 1 below shows the axial length (in millimetres) of the MPQHA foreach of a number of different combinations of ΔA (selected from therange of values 1-6 mm) and of ΔL (selected from the range of values 3to 12 mm). TABLE 1 ΔA (mm) ΔL (mm) 1 2 3 4 5 6 3 58.301 42.233 30.102 463.382 49.571 38.944 30.102 5 66.72 54.606 45.033 37.067 30.102 6 69.08458.301 49.571 42.233 35.835 7 70.847 61.135 53.11 46.285 40.328 8 72.21363.382 55.957 49.571 43.964 38.944 9 73.303 65.207 58.301 52.299 46.99542.233 10 74.193 66.72 60.267 54.606 49.571 45.033 11 74.932 67.99561.94 56.584 51.791 47.453 12 75.558 69.084 63.382 58.301 53.727 49.571

As can be seen from this Table the MPQHAs have axial lengths which areall less than that of the reference PQHA, regardless of the combinationof values ΔA, ΔL chosen.

FIG. 2(a) shows a plot of antenna return loss a function of frequency fobtained for the reference PQHA using an HP8510A network analyser (NWA).This plot shows that the first resonance frequency occurs at 2 GHz witha bandwidth of about 90 MHz, which is particularly desirable for mobilecommunications applications. The input impedance Z_(in) at 2 GHz wascalculated to be 61.7037-j 27.0820Ω. FIG. 2(b) shows the raditionpattern obtained from the reference PQHA at 2 GHz.

FIG. 3 shows plots of resonant frequency F as a function of ΔA obtainedfor the MPQHA for the different combinations of ΔA, ΔL presented inTable 1. This Figure shows that whereas the different MPQHAimplementations all have axial lengths that are less than that of thereference PQHA, their resonant frequencies are all higher than 2 GHzobtained for the reference PQHA, and are typically in the range 2.3 GHzto 2.4 GHz. The range of resonant frequencies obtained for the differentMPQHA implementations is relatively small, even though their axiallengths span a relatively wide range. This is because thecapacitance/unit length and the inductance/unit length both vary as afunction of ΔA and ΔL.

It was found that the MPQHA implementations investigated had generallylower resonant frequencies for larger values of ΔA (typically largerthan 2 mm) than the resonant frequencies obtained using an equivalentconventional meander line dipole (MDA). These lower frequencies atlarger values of ΔA are attributable to mutual coupling between oppositefilament elements of the antenna which does not, of course, occur in aMDA. Therefore, the MDQHA can offer a significant advantage over a MDA.

The results provided in Table 1 are grouped according to axial lengthand a meander geometric parameter β, where$\beta = {\frac{\Delta\quad A}{\Delta\quad L}.}$

The different groupings are presented in Table 2 along with the resonantfrequency for each combination of ΔA,ΔL represented in the Table as“MPQHA a-1”, where a is the value of ΔA and 1 is the value of ΔL. Also,included in each grouping is the resonant frequency of a PQHA having thesame axial length. TABLE 2 Group 1 Axial Length = 30.102 mm and β = 1Name PQHA MPQHA 3-3 MPQHA 4-4 MPQHA 5-5 Freq. (GHz) 3.525 3.07 2.832.51725 Group 2 Axial Length = 38.944 mm and β = 0.75 Name PQHA MPQHA3-4MPQHA6-8 Freq. (GHz) 3.1775 2.86475 2.17 Group 3 Axial Length = 42.233mm and β = 0.667 Name PQHA MPQHA2-3 MPQHA4-6 MPQHA6-9 Freq. (GHz)3.07325 2.8995 2.552 2.30875 Group 4 Axial Length = 45.033 mm and β =0.3 Name PQHA MPQHA3-5 MPQHA6-10 Freq. (GHz) 2.93425 2.57 2.30875 Group5 Axial Length = 49.571 mm and β = 0.5 MPQHA MPQHA MPQHA MPQHA MPQHAName PQHA 2-4 3-6 4-8 5-10 6-12 Freq. (GHz) 2.79525 2.6215 2.552 2.378252.37825 2.274 Group 6 Axial Length = 54.606 mm and β = 0.4 Name PQHAMPQHA2-5 MPQHA4-10 Freq. (GHz) 2.58675 2.552 2.30875 Group 7 AxialLength = 58.301 mm and β = 0.333 MPQHA MPQHA MPQHA MPQHA Name PQHA 1-32-6 3-9 4-12 Freq. (GHz) 2.552 2.48 2.37825 2.30875 2.2742 Group 8 AxialLength = 63.382 mm and β = 0.25 Name PQHA MPQHA1-4 MPQHA2-8 MPQHA3-12Freq. (GHz) 2.37825 2.37825 2.3435 2.23925 Group 9 Axial Length = 66.720mm and β = 0.2 Name PQHA MPQHA1-5 MPQHA2-10 Freq. (GHz) 2.274 2.2742.274 Group 10 Axial Length = 69.084 mm and β = 0.167 Name PQHA MPQHA1-6MPQHA2-12 Freq. (GHz) 2.274 2.274 2.23925

These tabulations clearly demonstrate that the majority of MPQHAimplementations (i.e. those having β values greater than about 0.25),resonate at frequencies that are lower than the resonant frequency of aPQHA having the same axial length. Also, it will be seen that axiallength of the MPQHA decreases as the value of β increases.

None of the MPQHA implementations listed in Table 2 resonantes at 2 GHz,required for some mobile communications applications. Therefore, forsuch applications, the design parameters need to optimised to provide aMPQHA which resonates at or very close to 2 GHz and yet has an axiallength much smaller than that of the reference PQHA.

The MPQHA implementation in Group 2 of Table 2 having the value ΔA=6 mmand the value ΔL=8 mm was chosen for optimisation because the axiallength (38.944 mm) and resonant frequency (2.17 GHz) are both relativelysmall.

Three different optimisation methods were considered.

The first optimisation method consists of increasing only the totallength L_(fil) of each filament by from 5% to 15%. Table 3 shows howincreases by 5%, 10% and 15% effect the resonant frequency and axiallength of the MPQHA. TABLE 3 Name MPQHA 6-8 MPQHA 6-8 MPQHA 6-8 MPQHA6-8 Percentage 0 5 10 15 increase (%) Element 89.315 93.78075 98.2465102.71225 length (mm) Axial length 38.944 42.233 45.428 48.546 (mm)Freq. (GHz) 2.17 2.17 2.18 2.04

An increase of L_(fil) by 15% has the effect of reducing the resonantfrequency of the antenna to 2.05 GHz, but at the expense of axial lengthwhich increases to 48.546 mm. The operating bandwidth of the optimisedMPQHA is 130 MHz.

FIG. 4(a) shows a plot of antenna return loss as a function of frequencyobtained using the optimised MPQHA, FIG. 4(b) shows the radiationpattern obtained from the optimised MPQHA at four different frequenciesand FIG. 4(c) shows radiation patterns obtained from the optimised MPQHAand from the reference PQHA at 2 GHz.

From FIG. 4(b) it can be seen that the optimised MPQHA also radiateswith greater efficiency at frequencies higher than 2 GHz, and it can beseen from FIG. 4(c) that the optimised MPQHA radiates with slightly lessefficiency than the reference PQHA, but, of course, has a much reducedaxial length, the size reduction factor α being 41.5%.

The second optimisation method consists of varying the value of ΔA whilethe value of ΔL is kept constant. As already explained, the value of ΔAhas an upper. limit ΔA_(max). It was found that no significant reductionof resonant frequency could be achieved by this method within theconstraints imposed by the antenna geometry.

The third optimisation method consists of varying the value of ΔL whileΔA is kept constant. This method has the advantage that the axial lengthof the antenna can be kept constant (at 38.944 mm), even though thevalue of ΔL is varied.

Table 4 shows the resonant frequency obtained for different values of ΔLin the range 3 mm to 10 mm. TABLE 4 MPQHA MPQHA MPQHA MPQHA MPQHA MPQHAMPQHA Name 6-8 6-8 6-8 6-8 6-8 6-8 6-8 ΔA (mm) 6 6 6 6 6 6 6 ΔL (mm) 3 45 6 7 9 10 β 2 1.5 1.2 1 0.857 0.667 0.6 Element 147 124 110 96 91 81 81length (mm) Axial 38.944 38.944 38.944 38.944 38.944 38.944 38.944length (mm) Freq. 1.90 2.03 2.06 2.13 2.16 2.29 2.28 (GHz)

Clearly, the optimum values of ΔL are 4 mm (giving a resonant frequencyof 2.03 GHz) and 5 mm giving a resonant frequency of (2.06 GHz). Theoperating bandwidth for both of these implementations is 190 MHz.

FIG. 5(a) shows plots of antenna return loss as a function of frequencyobtained for these two values of ΔL, and FIG. 5(b) shows radiationpatterns obtained for the two values of ΔL and for the reference PQHA,all at 2 GHz.

As can be seen from FIG. 5(b) both optimised MPQHAs radiate lessefficiently than the reference PQHA. Also, by comparing FIGS. 5(b) and4(b) it can be seen that the optimised MPQHA's obtained using the thirdoptimisation method radiate less efficiently than the optimised MPQHAobtained using the first optimisation method. However, the thirdoptimisation method gives a size reduction factor α of 53% which is muchhigher than that obtained using the first optimisation method.

The inventors have also found that there is some advantage in reducingthe track width w. If the track width w is reduced, the radius r of theMPQHA can also be reduced without neighbouring filaments overlapping.Also, a reduced track width w enables the value of ΔA to be reducedgiving a higher value β and a further reduction in axial length.

The resonant frequencies given in Table 4 above were all measured usingMPQHAs having a track width of 2 mm. The inventors have found that byreducing the track width to 1 mm there is no significant change ofresonant frequency, at least for the MPQHAs having the values ΔL 3 mm, 4mm and 5 mm. However, in each case the operating bandwidth is narrower.

It will be apparent from the foregoing that it is possible to optimiseone or more geometric parameters of the MPQHA to give a significantreduction in axial length as compared with a refernce PQHA and a desiredresonant frequency.

It will be appreciated that the invention is not restricted to thesquare waveform meander pattern; other periodic meander patterns can beused, including rectangular waveform patterns having mark-to-spaceratios greater or less than unity.

The MPQHAs that have been described are all designed to operate within asingle frequency band (centred on 2 GHz, for example). However, for someapplications an antenna having a multi-band operation is needed.

An example of this is an antenna for a dual band mobile communicationssystem which is intended to operate in accordance with both the DCS 1800and the UMTS standards. The frequency ranges required for thisapplication are as follows: DCS 1800 (Uplink) 1710 MHz to 1785 MHz DCS1800 (Downlink) 1805 Mhz to 1880 MHz UMTS (Uplink) 1920 MHz to 1980 MHzUMTS (Downlink) 2100 MHz to 2170 MHz

In a further embodiment of the invention, a microstrip spur-line bandstop filter is incorporated in each filament of a MPQHA at one end. Aswill be explained, the effect of the band stop filter is to create therequired dual band operation.

FIG. 6 shows a planar representation of the MPQHA filaments, eachincorporating a microstrip spur-line band stop filter F_(BS). FIG. 7 isa schematic view of a section of filament track 10 containing the bandstop filter, and for clarity of illustration the square-waveform meanderpattern has been omitted from this Figure.

Referring to FIG. 7, the microstrip spur-line band stop filter F_(BS)consists of a coupled pair of microstrip lines connected together at oneend and open circuit at another end. As described in “Design ofmicrostrip spur-line band stop filters”, Bates, R. N. IEEE Microwaves,Optics and Acoustics, Vol 1, No. 6, pp 209-214, the centre frequencyf_(o) of the band stop filter is related to the length a of the spurline 11 and to the gap b between the spur line 11 and the track 10 bythe expression$a = {\frac{2.997925 \times 10^{8}}{f_{o}\sqrt{K_{effo}}} - b}$where a and b are expressed in metres, f_(o) is expressed in Hz andK_(effo) is the odd mode effective dielectric constant.

In this embodiment, the MPQHA has the optimum geometric parameters asdetermined by the previously described third optimisation method i.e. ΔA  6 mm ΔL   4 mm N  0.75 r   7 mm L_(fil)  124 mm Axial length 38.944Resonant Freq (F) 2.03 GHz

In order to accomplish the required dual band operation the followingband stop filter parameters were used a   27 mm b  0.5 mm c   2 mm d0.75 mm e  0.5 mm

FIG. 8 a shows a plot of antenna return loss as a function of frequencyobtained for the MPQHA with and without the microstrip spur-line bandstop filter.

As can be seen from this Figure, the effect of the bandstop filter is toeliminate the resonant frequency at 2.03 GHz and to create two newresonant frequencies at 1.88 GHz and 2.17 GHz, giving rise to a lowerfrequency band and an upper frequency band respectively. The lowerfrequency band has an operating bandwidth of 110 MHz (extending from1.84 GHz to 1.95 GHz) and the upper frequency band has an operatingbandwidth of 100 MHz (extending from 2.12 GHz to 2.22 GHz). Thus, theeffect of the bandstop filter is to create a dual band MPQHA referred tohereinafter as DB-MPQHA.

FIG. 8 b shows the radiation patterns for the DB-MPQHA at each resonantfrequency (i.e. at 1.88 GHz and 2.17 GHz) and this Figure also shows theradiation pattern for the MPQHA (without the bandstop filter) at theresonant frequency 2 GHz.

The gain difference between the MPQHA and the DB-MPQHA at 1.88 GHz atelevation angle 0° is only 0.6 dB and the gain difference between theMPQHA and the DB-MPQHA at 2.17 GHz at elevation angle 0° is slightlyhigher at 1.1 dB.

FIG. 9 shows the positions of the afore-mentioned DCS 1800 and UMTSfrequency ranges superimposed on the plot of antenna return loss for theDB-MPQHA presented in FIG. 8 a. This demonstrates that the dual bandantenna operates in the lower frequency band for DCS 1800 and for theuplink of UMTS and operates in the upper frequency band for the downlinkof UMTS.

FIG. 10 shows radiation patterns obtained for the DB-MPQHA at fourdifferent frequencies. This Figure shows that the DB-MPQHA radiates moreefficiently in the frequency ranges required for UMTS than in thefrequency ranges required for DCS 1800. However, the radiation patternsare substantially the same at all frequencies which suggests that thereduction in efficiency at lower frequencies is due to poor matching,and it is believed that this problem can be resolved using an adaptivematching technique as described in PCT Publication No. WO99/41803.

It will be understood that although the microstrip spur-line band stopfilter has been described with reference to a meander printedquadrifilar helix antenna, this is not the only application of the bandstop filter. Alternatively, a microstrip spur-line band stop filtercould be applied to an otherwise conventional printed quadrifilar helixantenna (PQHA) to provide a required dual band operation.

Thus, in a further embodiment, a microstrip spur-line band stop filterwas incorporated in each filament of a PQHA having the followinggeometric parameters: Axial length  96 mm L_(fil) 102 mm r  7 mm Trackwidth  2 mm N 0.75 Resonant Freq (F)  1.8 GHz

The values of the band stop filter parameters were the same as thoseused for the MPQHA described earlier, except for the value of theparameter a. In fact, three different values of a were investigated(a=15 mm, 21 mm, 31 mm); however, only the value a=31 mm had asignificant effect. FIG. 11 shows plots of antenna return loss as afunction of frequency for the S₁₁ mode for the PQHA with and without themicrostrip spur-line band stop filter. As can be seen from this Figurethe effect of the band stop filter when a=31 mm is to eliminate theresonant frequency at 1.8 GHz and create new resonant frequencies at1.70 GHz and 1.98 GHz giving rise to upper and lower frequency bands,respectively.

Although the foregoing embodiments have all been described withreference to quadrifilar helix antennas, it will be understood that theinvention is also applicable to multifilar helix antennas having morethan four helical antenna filaments.

It will be appreciated that any of the described multifilar helixantennas may be used in, and is particularly well suited to, an adaptivemultifilar antenna arrangement as described in International PublicationNos. WO 99/41803 and WO 01/18908.

1. A multifilar helix antenna comprising, a plurality of helical antennafilaments spaced apart from each other at regular intervals about alongitudinal axis of the antenna, each said helical antenna filamenthaving a meander along its length.
 2. A multifilar helix antenna asclaimed in claim 1 wherein each said helical antenna filament has aperiodic said meander along its length.
 3. A multifilar helix antenna asclaimed in claim 2 wherein said periodic meander has a rectangularwaveform pattern.
 4. A multifilar helix antenna as claimed in claim 3wherein said rectangular waveform pattern is a square waveform pattern.5. A multifilar helix antenna as claimed in claim 4 wherein each saidhelical antenna filament comprises a track formed on an outer surface ofa cylindrical substrate,
 6. A multifilar helix antenna as claimed inclaim 5 wherein said track is formed by printing.
 7. A multifilar helixantenna as claimed in claim 5 wherein each periodic element of saidsquare waveform pattern has a length 2ΔL, the height of the squarewaveform pattern is ΔA and the ratio${\beta = {\frac{\Delta\quad A}{\Delta\quad L} \geq 0.25}},$ whereΔA=W−w, W is the overall width of each helical antenna element and w isthe width of the track.
 8. A multifilar helix antenna as claimed inclaim 7 wherein said ratio β is in the range from 0.25 to 1.0.
 9. Amultifilar helix antenna as claimed in claim 1 being a quadrifilar helixantenna.
 10. A multifilar helix antenna as claimed in claim 1 whereineach said helical antenna filament incorporates a band stop filter forenabling multi-band operation.
 11. A multifilar helix antenna as claimedin claim 10 wherein each said band stop filter is a microstrip spur-lineband stop filter.
 12. A multifilar helix antenna as claimed in claim 10wherein each said helical antenna element comprises a printed track andsaid band stop filter is a microstrip spur-line band stop filter formedin the track.
 13. A multifilar helix antenna as claimed in claim 10wherein said band stop filter enables dual. band operation
 14. Amultifilar helix antenna as claimed in claim 13 wherein said dual bandoperation is suitable for the DCS 1800 and UMTS standards.
 15. Amultifilar helix antenna comprising a plurality of helical antennafilaments spaced apart from each other at regular intervals about alongitudinal axis of the antenna and wherein each said helical antennafilament incorporates a band stop filter for enabling multi-bandoperation.
 16. A multifilar helix antenna as claimed in claim 15 whereinsaid band stop filter is a microstrip spur-line band stop filter.
 17. Amultifilar helix antenna as claimed in claim 15 wherein each saidhelical antenna filament comprises a printed track and said band stopfilter is a microstrip spur-line band stop filter formed in the track.18. A multifilar helix antenna as claimed in claim 15 wherein said bandstop filter enables dual band operation.
 19. A multifilar helix antennaas claimed in claim 18 wherein said dual band operation is suitable forthe DCS 1800 and UMTS standards.
 20. A multifilar helix antenna asclaimed in claim 15 being a quadrifilar helix antenna.
 21. A mobilecommunications terminal including a multifilar helix antenna as claimedin claim
 1. 22-23. (canceled)